Graphene-based composite film and preparation method and application thereof

Abstract

This invention relates to the field of electrodes, specifically to a graphene-based composite film, its preparation method, and its applications, aiming to solve the technical problems of poor electrochemical performance and short service life of existing graphene composite materials. The invention involves acidifying multi-walled carbon nanotubes to obtain modified carbon nanotubes; mixing these with graphene oxide, adding phytic acid and 4-aminopyridine for in-situ crosslinking, and then reducing and filtering with ascorbic acid to obtain a substrate film; placing the substrate film in a solution containing cobalt nitrate hexahydrate and 2-methylimidazole for hydrothermal reaction to obtain a composite film; finally, carbonizing under an inert atmosphere and using plasma bombardment in an argon-oxygen mixed atmosphere to create pores, thus obtaining the graphene-based composite film. This invention constructs a three-dimensional crosslinked porous conductive framework, combining confined growth and defect engineering to effectively prevent sheet aggregation and heavy metal leaching, significantly improving the specific capacitance and cycle life of the film.

Description

Technical Field

[0001] This invention relates to the field of electrodes, specifically to a graphene-based composite film, its preparation method, and its application. Background Technology

[0002] With the rapid development of technology, automobiles have become an indispensable tool in people's daily lives. However, the long-term, large-scale use of vehicles powered by non-renewable resources such as petroleum or diesel fuel will lead to problems such as petroleum resource shortages and environmental pollution. Electric vehicles powered by batteries, particularly lithium batteries, can avoid these drawbacks, leading to a surge in research on efficient and sustainable energy storage solutions. Supercapacitors are energy storage devices widely used in electric vehicles, smart grids, and energy storage systems. Supercapacitors have high power density, capable of releasing large amounts of electrical energy in a short time, typically completing rapid charging and discharging within seconds to minutes, making them suitable for applications with instantaneous power demands. Furthermore, compared to lithium-ion batteries, supercapacitors have a longer cycle life, capable of withstanding numerous charge-discharge cycles, typically reaching tens of thousands to hundreds of thousands.

[0003] Electrodes are the most important components of capacitors, and electrode materials play a decisive role in capacitor performance. Developing electrode materials with high capacitance, high power, and long cycle life is a crucial research direction. Carbon materials, due to their adjustable porosity, large specific surface area, high conductivity, and good stability, have become the main electrode materials currently being researched. Graphene films, with their excellent conductivity, high specific surface area, and good mechanical strength, are widely used in electrode materials and can be used to improve the energy density and power density of capacitors. To obtain graphene films with even better performance, the current industry consensus is to optimize the composite of graphene materials with other materials.

[0004] In summary, graphene-based composite films, as a material with high conductivity, large surface area, and strong mechanical strength, are widely used as electrode materials, but they suffer from poor electrochemical performance and short service life.

[0005] To this end, a graphene-based composite film, its preparation method, and its application are proposed. Summary of the Invention

[0006] The present invention aims to provide a graphene-based composite film, its preparation method, and its applications. The invention involves acidifying multi-walled carbon nanotubes to obtain modified carbon nanotubes; mixing these modified nanotubes with graphene oxide, adding phytic acid and 4-aminopyridine for in-situ crosslinking, and then reducing and filtering with ascorbic acid to obtain a substrate film; placing the substrate film in a solution containing cobalt nitrate hexahydrate and 2-methylimidazole for a hydrothermal reaction to obtain a composite film; finally, carbonizing under an inert atmosphere and bombarding with plasma in an argon-oxygen mixed atmosphere to create pores, thus obtaining the graphene-based composite film. This invention constructs a three-dimensional crosslinked porous conductive framework, combining confined growth and defect engineering to effectively prevent sheet aggregation and heavy metal leaching, significantly improving the specific capacitance and cycle life of the film.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a graphene-based composite film includes the following steps: Graphene oxide and modified carbon nanotubes were mixed, and phytic acid solution and 4-aminopyridine were added for in-situ chemical cross-linking. The substrate film was obtained by reduction with ascorbic acid. The substrate film was then subjected to in-situ MOF growth, and 2-methylimidazole was added followed by hydrothermal reaction to obtain a composite film. The composite film was carbonized in an inert atmosphere and bombarded with plasma to obtain a graphene-based composite film. The modified carbon nanotubes were obtained by acidification treatment of multi-walled carbon nanotubes.

[0008] Preferably, the modified carbon nanotubes are prepared by adding 20-30 parts of multi-walled carbon nanotubes to a mixture of concentrated sulfuric acid (98% by mass) and concentrated nitric acid (65%-68% by mass) in a volume ratio of 3:1 (1000 parts). The mixture is placed in a water bath at 80°C, refluxed, and magnetically stirred for 5-7 hours. After the reaction is completed, the mixture is washed with a large amount of deionized water until the pH of the filtrate is neutral. Then, it is dried in a vacuum drying oven at 60°C for 12 hours to obtain the modified carbon nanotubes.

[0009] Preferably, the preparation method of the substrate film is as follows: 95-105 parts of graphene oxide powder and 15-25 parts of modified carbon nanotubes are added to 2000 parts of deionized water and ultrasonically dispersed at 500W power for 1h to obtain a uniform suspension; 12-17 parts of phytic acid solution (70% aqueous solution) and 18-22 parts of 4-aminopyridine are added to the suspension in sequence, the temperature is raised to 60℃ and stirred continuously for 2h to obtain a crosslinking system; then 50 parts of ascorbic acid (VC) are added, the reaction temperature is raised to 90℃ and maintained for 3h; the mixture after reaction is vacuum filtered through a microporous filter membrane (pore size of 0.22μm, material of hydrophilic polytetrafluoroethylene) and vacuum dried at 60℃ for 24h, and the substrate film is obtained after peeling off the filter membrane.

[0010] Preferably, the composite film is prepared as follows: the substrate film is immersed in a methanol solution containing 140-160 parts of cobalt nitrate hexahydrate in 3000 parts of methanol, and left to stand for 2 hours to adsorb cobalt ions by chelation of phytic acid, thus obtaining a premixed solution; 295-305 parts of 2-methylimidazole are dissolved in a methanol solution in 2000 parts of methanol and slowly added dropwise to the premixed solution to obtain a mixed solution; the mixed solution is transferred to a hydrothermal reactor lined with polytetrafluoroethylene (the filling degree is controlled at 60%-80%), and reacted at 120°C for 4-8 hours. After the reaction is completed, the film is naturally cooled, the film surface is rinsed with methanol to remove free crystals that are not grown in situ, and then dried at 80°C to obtain the composite film.

[0011] Preferably, the preparation method of graphene-based composite film is as follows: the composite film is placed in a tube furnace and heated to 590-610℃ at a slow heating rate of 2℃ / min under a pure argon (Ar) atmosphere, and held at that temperature for 2h; the furnace temperature is lowered to 300℃, the protective gas is switched to a mixture of argon / oxygen (volume ratio 95:5), and a low-temperature plasma generator (power 50W, working pressure maintained at 10-50Pa, mixed gas flow rate set to 50-100sccm) is turned on and processed for 20-40min; the film is then naturally cooled to room temperature to obtain the graphene-based composite film.

[0012] An application of a graphene-based composite film, which is used in electrode materials.

[0013] A graphene-based composite film is prepared from raw materials including graphene oxide, modified carbon nanotubes, phytic acid solution, 4-aminopyridine, ascorbic acid and 2-methylimidazole.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention introduces modified multi-walled carbon nanotubes treated with mixed acid into graphene oxide sheets, and utilizes the electrostatic interaction and hydrogen bonding between the phosphate groups of phytic acid and the amino groups of 4-aminopyridine to form a supramolecular physical cross-linked pre-assembled network. The modified carbon nanotubes not only increase hydrophilicity but also act as conductive spacers interspersed between graphene sheets, effectively preventing severe stacking of graphene during charging and discharging. This three-dimensional cross-linked framework significantly reduces the internal resistance of the electrode, providing a high-speed channel for the rapid transport of electrons and ions, and substantially improving the electrochemical performance of the material from a physical mechanism perspective.

[0015] 2. This invention cleverly utilizes the abundant phosphate groups in phytic acid molecules as anchors to strongly chelate and adsorb cobalt ions in cobalt nitrate hexahydrate. Subsequently, 2-methylimidazole is added to guide the in-situ confined growth of MOF within the pores of the thin film. This strong chemical bonding ensures that the subsequently generated metal oxide is firmly embedded and fixed on the carbon substrate, preventing the dissolution and loss of active materials into the electrolyte, and solving the problem of rapid capacity decay of capacitors or batteries caused by heavy metal leaching.

[0016] 3. This invention employs a gradient treatment process combining inert gas carbonization with low-temperature plasma bombardment of argon-oxygen mixed gas. This process gently oxidizes MOF-derived cobalt particles in situ without damaging the overall porous carbon framework of the film, and creates oxygen vacancies in its lattice. These abundant oxygen vacancies not only greatly improve the electronic conductivity inside the metal oxide, but also provide extremely rich pseudocapacitive active sites for electrolyte ions, thus solving the problem of poor electrochemical performance of traditional high-crystallinity metal oxides.

[0017] 4. The phytic acid and 4-aminopyridine introduced in this invention during the preparation of the substrate film not only retain the cross-linked framework during the subsequent high-temperature carbonization process, but also uniformly dope phosphorus and nitrogen elements into the carbon lattice of graphene and carbon nanotubes. The synergistic doping of N and P atoms effectively regulates the surface polarity and electron cloud distribution of the carbon material, greatly improving the interfacial wettability between the electrode material and the electrolyte, allowing ions to penetrate deeper into the film; at the same time, the introduction of heteroatoms can also contribute some pseudocapacitance, working synergistically with metal oxides to further enhance the overall electrochemical performance.

[0018] 5. During long-term charge-discharge processes, metal oxides inevitably undergo drastic volume expansion and contraction, leading to electrode pulverization. In this solution, cobalt-containing nanopolyhedra and nitrogen-doped carbon networks (rigid) derived from MOFs under an inert atmosphere are completely encapsulated and confined within a flexible carbon nanotube-graphene cross-linked film. This flexible substrate provides ample buffer space for the volume changes of the metal oxides, absorbs deformation stress, and ensures that the electrode maintains structural integrity and does not collapse even when subjected to thousands of high-current impacts, thus giving the composite film an exceptionally long service life. Attached Figure Description

[0019] Figure 1 The graph shows the mass specific capacitance test results of Embodiments 1-5 of the present invention. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a graphene-based composite film, its preparation method, and its application. The technical solution is as follows: Example 1 25 parts of multi-walled carbon nanotubes were added to a mixture of concentrated sulfuric acid (98% by mass) and concentrated nitric acid (65%-68% by mass) in a volume ratio of 3:1 (1000 parts). The mixture was placed in a water bath at 80°C, refluxed, and magnetically stirred for 6 hours. After the reaction was completed, the mixture was washed with a large amount of deionized water until the pH of the filtrate was neutral. Then, it was dried in a vacuum drying oven at 60°C for 12 hours to obtain modified carbon nanotubes.

[0022] 100 parts of graphene oxide powder and 20 parts of modified carbon nanotubes were added to 2000 parts of deionized water and ultrasonically dispersed at 500W for 1 h to obtain a uniform suspension. 15 parts of phytic acid solution (70% aqueous solution) and 20 parts of 4-aminopyridine were added to the suspension sequentially, and the temperature was raised to 60℃ and stirred continuously for 2 h to obtain a crosslinked system. Subsequently, 50 parts of ascorbic acid (VC) were added, and the reaction temperature was raised to 90℃ and maintained for 3 h. The resulting mixture was vacuum filtered through a microporous membrane (pore size 0.22 μm, made of hydrophilic polytetrafluoroethylene) and vacuum dried at 60℃ for 24 h. After peeling off the filter membrane, the substrate film was obtained. The substrate film was immersed in a 3000-part methanol solution containing 150 parts of cobalt nitrate hexahydrate and allowed to stand for 2 hours. The cobalt ions were adsorbed by the chelating effect of phytic acid to obtain a premixed solution. 300 parts of 2-methylimidazole were dissolved in a 2000-part methanol solution and slowly added dropwise to the premixed solution to obtain a mixed solution. The mixed solution was transferred to a hydrothermal reactor lined with polytetrafluoroethylene and reacted at 120°C for 6 hours. After the reaction was completed, the film was naturally cooled, and the film surface was rinsed with methanol to remove free crystals that were not grown in situ. The film was then dried at 80°C to obtain a composite film. The composite film was placed in a tube furnace and heated to 600°C at a slow heating rate of 2°C / min under a pure argon (Ar) atmosphere, and held at that temperature for 2 hours. The furnace temperature was then lowered to 300°C, the protective gas was switched to a mixture of argon and oxygen (volume ratio 95:5), and a low-temperature plasma generator (power 50W, working pressure maintained at 10-50Pa, mixed gas flow rate set to 50-100sccm) was turned on and processed for 30 minutes. The film was then allowed to cool naturally to room temperature to obtain the graphene-based composite film.

[0023] Examples 2-5 follow the same preparation method and parameters as Example 1, with specific differences shown in Table 1.

[0024] Table 1. Parameter variations in Examples 1-5

[0025] Comparative Example 1 follows the same preparation method and parameters as Example 1, except that the multi-walled carbon nanotubes are not acidified and are added directly.

[0026] Comparative Example 2 follows the same preparation method and parameters as Example 1, except that no modified carbon nanotubes are added.

[0027] Comparative Example 3 was prepared using the same method and parameters as in Example 1, except that phytic acid solution was not added.

[0028] Comparative Example 4 was prepared using the same method and parameters as in Example 1, except that 4-aminopyridine was not added.

[0029] Comparative Example 5 was prepared using the same method and parameters as in Example 1, except that 2-methylimidazole was not added.

[0030] Comparative Example 6 follows the same preparation method and parameters as Example 1, except that the step of cooling to 300°C and starting argon-oxygen plasma bombardment is omitted, while the other steps remain unchanged.

[0031] Comparative Example 7 follows the same preparation method and parameters as Example 1, except that after the hydrothermal reaction, it is directly dried and tested without undergoing 600°C argon high-temperature carbonization treatment.

[0032] Comparative Example 8 was prepared using the same method and parameters as in Example 1, except that hydrazine hydrate was used instead of ascorbic acid.

[0033] Comparative Example 9 follows the same preparation method and parameters as Example 1, except that the processing time in the plasma bombardment pore-forming stage is extended from 30 min to 120 min.

[0034] Experiment Example 1: Mass Specific Capacitance and Cyclic Stability Test Electrochemical measurements were performed using a two-electrode system in a CHI660C electrochemical workstation. The graphene-based composite film was cut into two pieces, each 1.1 cm × 1.1 cm in size, and immersed in a 1 mol / L KCl solution to absorb the electrolyte. The two electrodes were then separated using filter paper that had absorbed the electrolyte, and a platinum sheet was used as the current collector. Cyclic voltammetry curves at different scan rates and charge-discharge curves at different current densities were obtained within a voltage window of 0–1.0 V. The frequency range of electrochemical impedance spectroscopy was 10 Hz. -2 -10-5 The frequency was Hz, and the amplitude was 5mV. Tests were conducted at current densities of 1A / g and 5A / g, and the discharge current, discharge time, and voltage difference were recorded to calculate the specific capacitance.

[0035] The graphene-based composite film was laminated with copper foil to form a graphene-based composite film electrode. A 0.1 mol / L potassium chloride solution was prepared as the electrolyte, and the solution was thoroughly stirred to ensure uniformity. The graphene-based composite film electrode, platinum electrode, and saturated calomel electrode were connected to form a three-electrode system. The electrolyte was poured into the battery case, ensuring that the electrodes were completely submerged. The electrochemical workstation parameters were set as follows: charge / discharge current density of 1 A / g, charging cutoff voltage of 2.7 V, discharging cutoff voltage of 1.0 V, and cycle count of 10,000.

[0036] The results are shown in Table 2. The specific capacitance performance of Examples 1-5 is as follows: Figure 1 As shown.

[0037] Table 2. Mass-to-capacitance and cycle stability of Examples 1-5 and Comparative Examples 1-4

[0038] From Table 4 and Figure 1It can be observed that Comparative Example 1 did not perform acidification treatment on the multi-walled carbon nanotubes, which directly resulted in the lack of oxygen-containing functional groups such as carboxyl and hydroxyl groups on their surface. These untreated carbon nanotubes exhibited high hydrophobicity and severely aggregated in aqueous solution, failing to disperse uniformly and penetrate into the graphene oxide sheets. They also failed to form effective hydrogen bonds or chemical bonds with the crosslinking agent. This not only led to a significant increase in the interfacial contact resistance inside the composite film and a substantial decrease in the specific capacitance under high current (5A / g), but also resulted in a deterioration in the mechanical strength of the film. Under the stress impact of long-term charge and discharge, local structural collapse was prone to occur, leading to a decrease in the capacitance retention rate after 10,000 cycles. In Comparative Example 2, no modified carbon nanotubes were added, resulting in the loss of the crucial one-dimensional conductive spacer support. During reduction and long-term cycling, the two-dimensional graphene sheets underwent severe stacking and re-aggregation due to strong π-π interactions. This stacking of sheets greatly reduced the effective specific surface area of ​​the material and blocked the fast transport channels of electrolyte ions. Therefore, its rate performance deteriorated the most severely, with a decrease in specific capacitance at a high current density of 5 A / g. At the same time, the densification of the structure prevented the active material from reacting fully, and the cycling stability also decreased. Comparative Example 3, which did not include phytic acid solution, exposed the material's fatal flaws during long-term cycling. Phytic acid is not only a source of phosphorus doping, but more importantly, its abundant phosphate groups act as chelates and anchors cobalt ions before in-situ growth. Due to the lack of chemical anchoring by phytic acid, the MOF-derived metal oxide particles only adhere to the carbon framework through weak physical adsorption. During 10,000 cycles of repeated charge and discharge, the active metal material is prone to pulverization and dissolves in large quantities into the electrolyte, resulting in a precipitous drop in capacitance retention. In addition, the absence of phosphorus doping also reduces the pseudocapacitive active sites of the material, causing a significant decrease in its initial specific capacitance. Comparative Example 4 did not include 4-aminopyridine, which prevented the reaction between amino groups and phytic acid phosphate groups in the system. This prevented the formation of a robust phosphorus-nitrogen three-dimensional covalent cross-linked network between the graphene sheets and carbon nanotubes. At the same time, the film lost catalytically active pyridine nitrogen doping sites, which significantly reduced the surface polarity and hydrophilic wettability of the carbon skeleton. Electrolyte ions could not penetrate deeply into the electrode, resulting in an increase in electrode polarization resistance. Ultimately, its specific capacitance was significantly lower than that of the example, and due to the relatively loose network structure, its long-cycle stability was also negatively affected to a moderate degree.

[0039] Comparative Example 5 omitted 2-methylimidazole during the hydrothermal reaction stage, preventing the in-situ confined growth of MOF (ZIF-67). The lack of organic ligand framework support and template isolation meant that cobalt ions adsorbed on the film were prone to uncontrolled aggregation during hydrothermal processing, directly forming large free cobalt oxide / hydroxide particles. These micron-sized aggregates not only severely blocked the porous structure of the three-dimensional carbon framework, hindering electrolyte ion penetration, but also easily pulverized and detached during long-cycle volume expansion due to the lack of MOF-derived carbon buffering. Comparative Example 6 omitted the step of cooling to 300℃ and initiating argon-oxygen plasma bombardment for pore formation, meaning the final material lacked activation of oxygen vacancy defect engineering. The metal oxide formed by simple high-temperature carbonization had excessively high crystallinity, resulting in extremely low intrinsic electronic conductivity and a lack of active pseudocapacitive reaction sites on the surface. This lattice structure led to very slow ion insertion / extraction kinetics, significantly increasing charge transfer resistance. Comparative Example 7 was tested directly after the hydrothermal reaction, omitting the 600℃ argon high-temperature carbonization treatment. This resulted in the ZIF-67 failing to transform into a composite structure of nitrogen-doped porous carbon and cobalt nanoparticles. Although the uncarbonized MOF material has a high specific surface area, its intrinsic conductivity is extremely poor, classifying it as an insulator. This leads to a huge resistance barrier within the composite film, preventing efficient electron transport between the active material and the carbon substrate, resulting in severe electrode polarization. Comparative Example 8 used a strong reducing agent, hydrazine hydrate, instead of the mild ascorbic acid. While it could still reduce graphene oxide, the process was too violent. Hydrazine hydrate caused graphene oxide to rapidly deoxygenate within a very short time, instantly regaining its strong hydrophobicity. This resulted in severe spontaneous aggregation and layer stacking in the aqueous system. This rapid and disordered aggregation disrupted the slowly forming three-dimensional uniform cross-linked network, leading to a final film with low porosity, dense structure, and high internal stress. This not only limited the subsequent infiltration and loading of MOF precursors but also hindered ion diffusion in the electrolyte. Comparative Example 9 extended the plasma bombardment pore-forming process time from 30 minutes to an extreme 120 minutes, resulting in over-etching. Prolonged high-energy plasma bombardment not only created defects on the metal oxide surface but also shattered the intrinsic spline properties of graphene and carbon nanotubes. 2 The carbon hybrid framework network disrupts numerous conductive pathways in the thin film, severely reducing its mechanical strength.

[0040] Experimental Example 2: Specific Surface Area and Pore Volume Test The specific surface area and pore volume of the graphene-based composite film were measured using a Kubo (X1000) surface area analyzer. The graphene-based composite film was heated to 60°C and held in a vacuum drying oven for 12 hours to remove moisture and solvents. The sample was washed with deionized water or ethanol to remove surface contaminants and impurities. Before testing, the sample was placed in a nitrogen atmosphere to ensure no other gases were adsorbed on the sample surface. The sample was degassed in the testing instrument, and the obtained data were recorded and observed. The results are shown in Table 3.

[0041] Table 3. Specific surface area and pore volume of Examples 1-5, Comparative Examples 1-4 and Comparative Example 8

[0042] Table 3 shows that in Comparative Example 1, no acidification treatment was applied to the multi-walled carbon nanotubes. Due to the extremely strong hydrophobicity and lack of oxygen-containing functional groups on the surface of the original carbon nanotubes, they were extremely difficult to disperse in the aqueous system and experienced severe physical entanglement and aggregation. This resulted in the carbon nanotubes being unable to uniformly penetrate between the graphene oxide sheets to provide effective skeletal support. A large number of graphene sheets still stacked, causing a significant decrease in the specific surface area and pore volume of the composite film, which limited the effective penetration of electrolyte ions. In Comparative Example 2, no modified carbon nanotubes were added, resulting in the complete absence of one-dimensional conductive spacers in the composite film. During ascorbic acid reduction and subsequent heat treatment, the two-dimensional graphene sheets underwent extremely severe irreversible stacking and recombination due to strong van der Waals forces and π-π interactions. The originally completely open three-dimensional porous structure collapsed directly into a dense layered structure. Therefore, its specific surface area and pore volume were at a relatively low level among all groups. Comparative Example 3, lacking phytic acid solution, resulted in a deficiency of crucial crosslinking and chelating agents. On one hand, a robust chemical crosslinking network could not be formed between graphene and carbon nanotubes, leading to a loose overall framework that was prone to localized pore shrinkage during vacuum filtration and drying. On the other hand, the lack of pre-chelation confinement of cobalt ions by phytic acid caused the subsequently generated MOF (ZIF-67) crystals to agglomerate randomly on the film surface or in large pores. This large-particle blockage effect directly reduced the effective pore volume and specific surface area of ​​the film. Comparative Example 4, lacking 4-aminopyridine, prevented the reaction between amino and phosphate groups, thus hindering the construction of a phosphorus-nitrogen covalently crosslinked three-dimensional network. The carbon framework, maintained only by physical mixing and weak hydrogen bonds, was unable to resist the shrinkage caused by internal stress during the drastic physicochemical changes of hydrothermal reaction and high-temperature carbonization, resulting in significant pore collapse and structural densification. Comparative Example 8 used a strong reducing agent, hydrazine hydrate, instead of ascorbic acid. The violent reduction reaction caused a large number of oxygen-containing functional groups on the graphene oxide to be removed in a very short time. This sudden depolarization process caused the graphene sheets to instantly regain strong hydrophobicity and undergo rapid spontaneous aggregation and shrinkage in aqueous solution. Compared with the uniform three-dimensional porous network constructed by the slow and gentle reduction of ascorbic acid, the film structure generated by the reduction of hydrazine hydrate is extremely dense and contains a large number of dead pores (closed pores), which greatly reduces its specific surface area and pore volume.

[0043] Experimental Example 3: Conductivity and Heavy Metal Leachability Test The sample was dried at 80°C for 1 hour to ensure the graphene-based composite film was dry and flat. Metal electrodes were deposited at both ends of the film to ensure good contact. The sample was placed in a four-probe measuring instrument, and the power supply and current measuring device were connected. The power supply voltage was set to 0.5V, and the current flowing through the sample was recorded to calculate the conductivity.

[0044] After 10,000 charge-discharge cycles of the graphene-based composite film electrode, an electrolyte sample was taken for analysis. The extracted electrolyte was centrifuged to remove suspended solids, and the supernatant was then used for heavy metal analysis. The supernatant was filtered and placed in an atomic absorption spectrometer with a series of cobalt ion standard curve solutions ranging from 0 to 1 mg / L. The current was set to 10 mA, and the flame type was air-acetylene flame. The heavy metal concentration was calculated using the external standard method.

[0045] The results are shown in Table 4.

[0046] Table 4. Conductivity and heavy metal leaching of Examples 1-5 and Comparative Examples 5-9

[0047] As shown in Table 4, Comparative Example 5 did not include 2-methylimidazole, which prevented the formation of the ZIF-67 (MOF) precursor. During the hydrothermal process, cobalt ions directly agglomerated into micron-sized cobalt oxide or cobalt hydroxide particles, and the surface lacked the nitrogen-doped porous carbon layer derived from MOF. Due to the poor intrinsic conductivity of the bulk metal oxide and the failure to form a good rigid-flexible continuous conductive network with the carbon substrate, the conductivity of the film decreased significantly. More seriously, without the physical encapsulation and confinement effect of the MOF-derived carbon layer, the metal oxides free on the carbon skeleton surface were easily eroded by the electrolyte and pulverized and peeled off during 10,000 charge-discharge cycles, resulting in a surge in heavy metal leaching and severely shortening the electrode's lifespan. Comparative Example 6 omitted the step of cooling to 300℃ and initiating argon-oxygen plasma bombardment, resulting in a metal oxide with extremely high crystallinity and a perfect lattice but lacking oxygen vacancies. Oxygen vacancies act as shallow donors in semiconductor physics, significantly increasing the carrier concentration of the system. Due to the lack of this crucial defect engineering activation, the electronic conduction resistance inside the active material increases sharply, leading to a decrease in the overall conductivity of the thin film. Comparative Example 7 did not undergo high-temperature carbonization at 600℃ using argon after the hydrothermal reaction, which is the core reason for its complete performance collapse. Uncarbonized ZIF-67 is an organic-inorganic hybrid material, essentially an electrical insulator. This results in the composite film being filled with high-resistance insulating islands, causing a precipitous drop in conductivity. Furthermore, the MOF structure without high-temperature carbonization is extremely unstable during electrochemical cycling in strong acid or strong alkaline electrolytes. The framework rapidly undergoes chemical degradation and structural collapse, directly exposing and dissolving large amounts of cobalt ions in the electrolyte, leading to a high degree of heavy metal leaching. In Comparative Example 8, hydrazine hydrate was used instead of ascorbic acid as a reducing agent. Its violent reducing effect destroyed the uniform dispersion of the system, resulting in severe local hydrophobic aggregation of graphene oxide. This uneven aggregation interrupted the continuous conductive pathway that could be bridged by carbon nanotubes, causing a decrease in the conductivity of the film. At the same time, because the substrate framework became dense and uneven, cobalt ions could not be uniformly confined inside the pores. A large number of cobalt particles could only accumulate on the outer surface of the film. These surface cobalt particles, which were not effectively protected by the internal three-dimensional network, were prone to detachment during charge-discharge cycles, resulting in an increase in the dissolution of heavy metals. Comparative Example 9 extended the plasma bombardment time from 30 min to 120 min, resulting in severe etching of the film. The long-term bombardment of high-energy plasma completely destroyed the hybrid carbon conjugated framework of graphene and carbon nanotubes, burning through the originally complete highly conductive macromolecular network into fragments, leading to a sharp decrease in conductivity. More fatally, the excessive etching stripped away the carbon protective layer that originally wrapped the cobalt nanoparticles, leaving the metal active material completely exposed. Under cyclic stress and the scouring of the electrolyte, these unprotected cobalt particles were lost in large quantities, and the dissolution rate of heavy metals increased significantly.

[0048] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a graphene-based composite film, characterized in that: Includes the following steps: Graphene oxide and modified carbon nanotubes were mixed, and phytic acid solution and 4-aminopyridine were added for in-situ chemical crosslinking. The substrate film was obtained by reduction with ascorbic acid. The substrate film was then subjected to in-situ MOF growth, and 2-methylimidazole was added followed by hydrothermal reaction to obtain a composite film. The composite film was carbonized in an inert atmosphere and bombarded with plasma to obtain the graphene-based composite film. The modified carbon nanotubes are obtained by acidification of multi-walled carbon nanotubes.

2. The method for preparing a graphene-based composite film according to claim 1, characterized in that: The modified carbon nanotubes are prepared by adding the multi-walled carbon nanotubes into a mixture of concentrated sulfuric acid and concentrated nitric acid, placing the mixture in a water bath, refluxing and magnetically stirring for 5-7 hours; after the reaction is completed, the mixture is washed with deionized water until the pH of the filtrate is neutral, and then vacuum dried to obtain the modified carbon nanotubes.

3. The method for preparing a graphene-based composite film according to claim 1, characterized in that: The substrate film is prepared as follows: graphene oxide powder and the modified carbon nanotubes are added to deionized water and ultrasonically dispersed to obtain a suspension; phytic acid solution and 4-aminopyridine are added sequentially to the suspension, and the temperature is raised to 60°C and stirred continuously to obtain a crosslinking system; then ascorbic acid is added, and the reaction temperature is raised to 90°C to maintain the reaction; the mixture after the reaction is vacuum filtered through a microporous membrane, vacuum dried, and the substrate film is obtained after peeling off the filter membrane.

4. The method for preparing a graphene-based composite film according to claim 1, characterized in that: The composite film is prepared as follows: the substrate film is immersed in a methanol solution containing cobalt nitrate hexahydrate, and after standing, a premix is ​​obtained; the 2-methylimidazole is dissolved in a methanol solution and added dropwise to the premix to obtain a mixed solution; the mixed solution is transferred to a hydrothermal reactor lined with polytetrafluoroethylene and reacted at 120°C for 4-8 hours. After the reaction is completed, the mixture is naturally cooled, rinsed with methanol, and dried to obtain the composite film.

5. The method for preparing a graphene-based composite film according to claim 1, characterized in that: The graphene-based composite film is prepared as follows: the composite film is placed in a tube furnace and heated to 590-610°C at a heating rate of 2°C / min under an argon atmosphere, and held at that temperature; the furnace temperature is lowered to 300°C, the protective gas is switched to a mixture of argon and oxygen, and the plasma generator is turned on for 20-40 minutes; the film is then naturally cooled to room temperature to obtain the graphene-based composite film.

6. An application of a graphene-based composite film, characterized in that: The graphene-based composite film is used in electrode materials, and the graphene-based composite film is prepared by the preparation method according to any one of claims 1-5.

7. A graphene-based composite film, characterized in that: The raw materials for preparing the graphene-based composite film include graphene oxide, modified carbon nanotubes, phytic acid solution, 4-aminopyridine, ascorbic acid, and 2-methylimidazole; the graphene-based composite film is prepared by the preparation method described in any one of claims 1-5.

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